1. Field of the Invention
The present invention relates to non-linear optical materials having second-order non-linear optical characteristics, a method of manufacturing the same and optical wavelength converters using the same.
2. Description of the Prior Art
Heretofore, KH.sub.2 PO.sub.4 (abbreviated as KDP), LiNbO.sub.3 (abbreviated as LN) and like materials have been used for laser beam wavelength converters and electro-optic modulators using Pockels effect. Recently, organic compounds having large optical nonlinearity as the crystals have been found, and study and development concerning them are being made.
In order for a crystallized organic compound to have second-order non-linear optical characteristics, it is necessary that the molecular second-order polarizability .beta. of the compound is relatively large and that the crystal structure is not centrosymmetric. For destroying the centrosymmetric structure, the following means can be used:
(1) Like the case of 2-methyl-4-nitroaniline (abbreviated MNA), substituent group (i.e., a methyl group in this case) are introduced to decrease the symmetry of the molecules.
(2) Like the case of methyl-(2,4-dinitrophenyl)aminopropanate, N-(5-nitro-2-pyridyl)-(S)-prolinol or like compound, an asymmetric carbon atoms is introduced to a molecule to decrease the symmetry of the molecules.
(3) To form an organic salt such as a trans-4'-dimethylamino-N-methyl-4-stilbazolium methylsulfate.
Well-known organic compounds having second-order non-linear optical characteristics and available as large crystals, are organic salts of L-arginine phosphate monohydrate (abbreviated as LAP) and so forth.
Crystal organic compounds having second order non-linear optical characteristics previously included those compounds which form molecular crystals. These compounds have molecules which are bonded together in the crystal by van der Waals bonds or hydrogen bonds. As the binding force is weak, and as the molecular symmetry is low, it is difficult to obtain a large crystal of this type of compound. Even if a large crystal can be obtained, subsequent mechanical processing (such as cutting or polishing to obtain an optical surface) is difficult or impossible because of low mechanical strength. Thus, it is impossible to obtain a surface having satisfactory optical characteristics. Problems are, therefore, presented when the crystal is processed into a device or element.
In ionic crystal organic compounds, such as salts, strong ionic bonds are formed in the crystal compared to van der Waals bonds or hydrogen bonds. Single crystals which are relatively large and have high mechanical strength, are obtainable. However, with the well-known ionic crystal organic compounds having second order non-linear optical characteristics, e.g., a trans-4'-dimethylamino-N-methyl-4-stilbazolium methylsulfate, the length of the conjugated .pi. electron system is very large. That is, the optical absorption maximum wavelength is comparable to that of the second harmonic of Nd:YAG or semiconductor laser beams. Therefore, wavelength converted light is absorbed by the material, and the second harmonic can not be efficiently obtained. The LAP has small optical non-linearity and particularly low efficiency of low power laser beam conversion. Therefore, it can not be used for the wavelength conversion of semiconductors or like laser beams.